1. Field of the Invention
The present invention relates generally to an optical distance measurement system and in particular to such a system which provides precision measurement using radio frequency modulated laser beams.
2. Background Art
In current electromagnetic distance measurement technology, lengths are determined based on the time it takes electromagnetic energy to travel from one end of a line to the other and return. This electromagnetic energy can be light, either visible or infrared; microwave; or radio wave which transmits energy having very long wavelengths. The electromagnetic energy, especially light and microwave, may be modulated. Generally, this equipment measures distances by comparing a line of unknown length to the known wavelength of the electromagnetic energy or the modulation wavelength and operates by measuring phase shift. The returned electromagnetic energy goes through a complete 360' phase change for every integer multiple of exactly one-half of the wavelength which separates the line's endpoints. When the line is not an integer multiple of one-half of the wavelength, the fractional part shows as a non-zero phase change which can be converted to distance.
Most current electronic distance measurement instruments, particularly interferometer devices that use unmodulated light, resolve the fractional wavelengths but do not count the absolute total number of full wave cycles which the returned electromagnetic energy has gone through. To resolve this unknown factor, some equipment (not based on interferometer principles) transmits additional modulated energy of lower frequency and hence longer wavelength as a coarse measurement.
One problem encountered with prior art modulated light, microwave or RF (that is non-interferometer equipment) measurement is that it is not accurate enough for precision range and object measurements such as encountered in machine tool work, because of the long wavelengths associated with practical RF frequencies. There is, therefore, difficulty in determining fine resolution between the phase output of the return and reference phases. Interferometer type distance measuring systems using unmodulated light have sufficient resolution, but there is the problem of aligning and resolving light wavefronts since an optical interferometer uses only the relationship of the light phase. The necessary high resolution phase measuring circuitry is difficult to implement at the much higher frequency of light and does not resolve ambiguities as required for absolute distance measurement.
Known Tellurometer equipment is used for surveying because it is able to measure distance along a beam of modulated light. However, the resolution of the Tellurometer type unit is a millimeter or so, which is suitable for land surveying, but not for machine tool or the like that requires resolution down to the order of 0.01 micron.
U.S. Pat. No. 3,779,645 to Nakazawa, et al, discloses a distance measuring device which uses an amplitude-modulated light wave to measure the phase difference between the transmitted and the received modulated wave to thereby determine distance. Intensity modulation is used to compensate for changes in the diminution of the transmit signal over the range to the distant target and return.
U.S. Pat. No. 3,950,100 to Keene, et al, discloses a laser heterodyne system in which received reflections of light are amplified in a laser. The amplified reflections are heterodyned with the light from the laser and the resulting beat frequency is detected with a photodetector.
U.S. Pat. No. 4,093,380 to White teaches an optical system which uses a three-wave heterodyne detector wherein two optical frequencies are supplied by the input signal so as not to require a local source of optical radiation. This system uses a sophisticated solid state three component mixer detector.
U.S. Pat. No. 4,403,857 to Holscher discloses a distance measuring device which uses a modulated light source wherein phase differences are compared to determine distance. This system is an example of the Tellurometer system.
U.S. Pat. No. 4,537,502 to Miller, et al, discloses a multiple discrete frequency device which comprises a modulated carrier frequency transmitter and a generator for generating at least two modulating signals. Two optical beams of coherent light having closely related frequencies are mixed in an optical mixer.
U.S. Pat. No. 4,715,706 to Wang discloses a laser Doppler range finder for use in measuring the displacement of a moving target.
U.S. Pat. No. 4,743,110 to Arnaud, et al, discloses a laser telemetry and Doppler measurement system which uses a periodic pulsed transmission laser.
U.S Pat. No. 4,774,653 to Sano et al. discloses a method and apparatus for measuring a distance to an object using two laser beams. Switched lasers are used to resolve ambiguity.
U.S. Pat. No. 4,916,536 to Kerr, et al, discloses an imaging range finder which has a transmitting section which directs radiation across an angular field of view by rotating a mirror. The receiving section also has a rotating mirror which collects any reflected radiation. Range is determined by radiation modulation.
R. Dandliker, R. Thalmann, D. Prongue, Optics Letters' May 1988, Vol. 13, No. 8, p. 339-341, discloses the generation of synthetic wavelengths to reduce the sensitivity or to extend the range of unambiguity in interferometric measurements.
C. C. Williams and H. K. Wickramasinghe, "Absolute Optical Ranging with 200 mm Resolution", Optics Letters, June 1, 1989, Vol. 14, No. 11, p. 542-544, discloses the combining of single and multiplexed wavelength interferometric measurements to achieve unambiguous distance measurement with nanometer resolution.
I. I. Adrianova, V. G. Vafiadi, V. V. Volkonsky, Z. V. Nesterova, Yu V. Popor, and A. F. Shilor, Design Principles of Phase Optical Rangefinders with Microwave-Modulated Radiation, p. 846-855, discloses microwave-modulated light used for distance measurement in optical rangefinders.